Actually, Next.js could also be used to talk to the PHP backend by proxying API calls. PHP is performing great these days and is high performance as well. The big issue is only: lot's of users are on shared servers where resources are shared, which could cause PHP to become slow. Running it on your own server and have the optimal settings shouldn't be too difficult 😛
As a React developer myself, I would also be interested to see if Flarum would ever be able to React.js or maybe even Preact.js (which has most of the React JS API's, but has a smaller footprint and is better performant). But, I'm used to Mithril as well so that's fine 🙂